I recently learned that osmosis occurs when there is a semipermeable membrane between pure water and a solution of water. The pure water then fills up the solution's side until, as my textbook put it,

"The pressure exerted by this column of solution counterbalances the pressure exerted by the water moving through the membrane from the pure water side."

My question is how does water just naturally want to move from one gradient to the other? Doesn't water move through the membrane both ways? Also, why does water "want" to go to the more concentrated side when the particles move at random?

In other words, why doesn't the flow of water molecules depend on the number of water molecules in the membrane instead of the concentration gradient?

  • $\begingroup$ There is water concentration, or rather activity, difference. More water molecules tries to get to the other side from the less concentrated solution. $\endgroup$
    – Poutnik
    Feb 19, 2021 at 14:02

1 Answer 1


The membrane has plenty of holes that are a little bit bigger than small molecules like water. Bigger molecules or bigger ions like usual solutes cannot go through these holes. Of course water has the tendency to cross the membrane both ways. When a water molecule hits a hole in the membrane from the pure water side, it can cross it. Nothing prevents it. On the other side, when a solute molecule hits a hole of the same membrane, it blocks the passage for a couple of microseconds. No water molecules can go through this hole from the solution side of the membrane during this time. So on average, more water will cross the holes of the membrane in the direction pure water to solution than backwards. Is it what you wanted to know ?

  • $\begingroup$ Water diffuses across the membrane in both directions. The higher concentration of water [AKA Activity] in the water with less solute means more collisions with the membrane more penetration into the membrane and more transport across the membrane. The process equilibrates when the water activities are the same on each side of the membrane. The solute does not "block the passage". Membranes, especially in reverse osmosis where pressure is used to reverse the natural osmosis, will coat, foul, and the membrane becomes nonpermeable. When this happens the holes are plugged. $\endgroup$
    – jimchmst
    Aug 14, 2022 at 0:44
  • $\begingroup$ Instead of saying "blocks the passage", you could just say that the rate of water getting to the holes is lower because the concentration of water is lower. If you make a molecular simulation, it does look like blocking a bit. Because two molecules can't be in the same place at the same time, one molecule always blocks the other in that sense. $\endgroup$
    – Karsten
    Dec 12, 2022 at 4:25

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